The present invention relates to a pipe made of synthetic resin and which is intended for underground use. Particularly, the invention relates to such a pipe having a high pressure resistance and which may be used for an underground water main, a sewer or drain pipe, a pipe for protecting electrical cables, or the like.
A conventional underground pipe of the same general type to which the invention pertains has been manufactured by a process illustrated in FIGS. 1 and 2 of the accompanying drawing. With reference first to FIG. 1, a reinforcing rib 04, used to form corrugations on the exterior surface of the pipe, is manufactured from corrugated tube 03 having corrugations extending perpendicular to the corrugations of the completed pipe a which is covered with an extruded coating 02. To form the pipe, the rib 04 is wound around the main body 01 of the pipe in a molten state, pressing the rib 04 into the material of the pipe body 01.
A pipe so constituted is advantageous in that it has a good overall strength and a sufficient resistance to the pressure of the soil in which the pipe is buried. Also, the pipe is light in weight and can be readily handled. On the other hand, the pipe is disadvantageous in that it is difficult to control how far the rib 03 sinks into the material of the pipe body 01 when the rib 04 is being wound around and fused into the material of the pipe body 01. Accordingly, often the rib 04 sinks too far into the pipe body 01, making the pipe weak in such areas.
To overcome this problem, the present inventor has proposed (in Japanese Utility Model Application No. 122,687/1984) a pipe construction in which the rib is manufactured of the same materials as the material of the body 01 and is formed with a semicircular cross section with the flat bottom of the rib fitted in an outwardly projecting portion formed on the outer wall of the pipe body. Although a pipe of this structure has been found to have a sufficient pressure withstanding strength, it is necessary to make the wall of the pipe body thicker than is otherwise desired, thereby making the pipe heavy as a whole, thus making the pipe difficult to handle. Particularly, the pipe lacks flexibility, a desirable property in pipes of this general type. Moreover, the pipe is costly in that a large amount of resin material is needed in its manufacture.